The high intensity sodium vapor lamps with which the invention is most useful comprise a slender tubular ceramic arc tube which is generally mounted in an outer vitreous envelope or glass jacket. The arc tube is made of light-transmissive refractory oxide material resistant to sodium at high temperatures, suitably high density polycrystalline alumina or synthetic sapphire. The tube contains a discharge supporting filling comprising sodium together with mercury for improved efficiency, along with a rare gas to facilitate starting. Thermionic electrodes are contained within the tube whose ends are sealed by closure members through which connections are made to the electrodes. The outer envelope which encloses the ceramic arc tube is generally provided at one end with a screw base to which the electrodes of the arc tube are connected.
The high pressure sodium vapor lamp contains an excess amount of sodium-mercury amalgam, that is it contains more amalgam than is vaporized when the lamp reaches a stable operating condition. By having an excess, the quantity supplied is made noncritical and some of the excess amalgam is used to replace any lost during the life of the lamp as it ages, for instance by electrolysis through the alumina walls.
The lamp voltage, that is the voltage drop across the arc tube during normal operation, is dependent upon the vapor pressure and the vapor pressure in turn is determined by the lowest temperature in the arc tube which is dependent upon the thermal balance. A preferred lamp design utilizes an externally projecting metal exhaust tube which is sealed off and provides a reservoir for excess sodium mercury amalgam external to the arc tube proper. This arrangement has the advantage of placing the excess amalgam in a location removed from the direct heat of the arc and of the electrodes, so that arc tube blackening as the lamp ages has a minimal effect on sodium vapor pressure and on lamp voltage. Also the use of an external reservoir facilitates close adjustment of the heat balance in the lamp. Another lamp design avoids the need for an exhaust tube by inserting the charge of sodium-mercury amalgam into an arc tube closed at one end. Then, while the closed end is cooled, the other end is sealed off in a chamber containing an atmosphere of the inert starting gas intended for the lamp. In such a lamp, the heat balance is planned to make one end or the other the cold spot and the excess amalgam collects mostly in the corners where the end cap or plug is joined to the ceramic body. In both designs, lamp voltage increases as the lamp ages, and the end of life is reached when the ballast can no longer sustain the arc across the high voltage drop prevailing.
In high pressure sodium lamp manufacture, dimensions of parts, material quality and processing are carefully controlled in order to maintain lamp voltage within specified limits. Nevertheless over 10% of such lamps produced in the plants of applicant's assignee currently must be reworked because the voltage of the finished lamp falls above or below the specified limits. Reworking in order to salvage lamps is expensive and time consuming. It has meant breaking the outer envelope or jacket, cutting the ceramic arc tube out from the old stem assembly, welding it to a new stem assembly, and adding or removing exhaust tube radiation shields to correct the heat balance. Alternatively, the heat balance may be modified by grit-blasting the exhaust tube or by painting chrome green paint on it. The reworked arc tube must be sealed into a new outer envelope which must then be evacuated and the lamp must be rebased and reseasoned.